Equipment advances and reduction in pricing have allowed 3D printing to become widely adopted in homes, schools, and industry as a fast, simpler, and often cheaper way to prototype and make custom end-use parts. Specifically, material extrusion 3D printing (also known as fused filament fabrication or fused deposition modeling), has emerged as a tool of choice for direct consumer use, larger scale production, and quick thermoplastic prototyping as it is the easiest to operate, produces the least waste, and provides the shortest turnaround time of conventional 3D printing technologies.
Many materials have been used to produce 3-D printed articles for a wide variety of end uses, from chocolate to collagen. Thermoplastic materials are especially well adapted for use with 3-D printers. Unfortunately, there have been few thermoplastics available that provide high chemical resistance, flame resistance, and good mechanical properties.
Some amorphous polymers like polyphenylsulfone (PPSU) have only a 3 percent elongation when printed. Nylons have higher elongation (˜30%) but poor chemical resistance and must be dried prior to printing. Much higher elongation, flexible thermoplastic polyurethanes are available but have poor chemical resistance and weathering resistance.
Fluoropolymers are known for having excellent chemical resistance and good mechanical integrity. Most fluoropolymers, such as fluorinated ethylene propylene (FEP), perfluoroalkoxy alkane (PFA), ethylene tetrafluoroethylene (ETFE), ethylene chloro-trifluoro ethylene (ECTFE), perfluoromethyl alkoxy polymer (MFA) have narrow processing windows (between Tm and degradation temp) that make 3D printing difficult. In addition they have high shrinkage on transition from the melt to a final solid, making warpage a real issue.
Polyvinylidene fluoride (PVDF) is a semi-crystalline fluoropolymer that exhibits adhesion and warpage issues when 3-D printed. PVDFs semi-crystalline nature (PVDF homopolymer is known to have up to 50-65% crystallinity), and its high volume shrinkage during solidification (15-40%), lead to a high propensity to shrink and warp during filament 3D printing. PVDF's low surface energy of generally 25 to 31 dyne-cm results in poor adhesion to most materials.
Another issue facing fused deposition modeling printed plastics in general is that due to the layer by layer deposition process, the printed parts have significant mechanical anisotropy with properties measured in the z direction significantly lower than those measured in the xy direction. The xy directions are parallel to the bottom build stage, while the z direction is perpendicular to the build stage. The process typically involves the deposition of a layer in the xy direction followed by another layer in the xy direction. The z direction is built up by layers being deposited on top of each other. As a result printed functional parts do not perform as well in the z direction (such as a ball joint snap fit printed vertically, as shown in the Examples).
There is a need for processes and/or formulations allowing for acceptable 3-D printing of crystalline fluoropolymer articles. Fluoropolymer articles are desirable in 3-D printing for their chemical resistance, durability, flame resistance, and mechanical properties. Fluoropolymers such as PVDF that can be printed with little warpage and part shrinkage are highly desired. Such materials could be used in the industry for prototyping and custom end-use parts where high chemical resistance, durability, and part integrity are needed.
Surprisingly, after much research, a semi-crystalline fluoropolymer and process conditions have now been developed, that can be used in a filament 3-D printing process with good resistance to shrinkage and warpage. Additionally, the fluoropolymer or fluoropolymer formulation provides high chemical resistance, high water resistance, hydrophobicity, thermal and UV resistance, high layer to layer adhesion, good use temperature, good mechanical properties, high Relative Thermal Index (RTI) rating (130-150° C.), flame resistance, high elongation to break, and high impact, with the mechanical strength and stiffness that varies with the comonomer level in the PVDF, formulation components, filler type/level, and printing conditions, 3-D articles and parts printed from these formulations and using the methods of the invention could be stiff or flexible, with improved print resolution, and approach the strength and elongation to break of injection molded fluoropolymer parts. Filament made of the composition of the invention, due to its excellent moisture resistance, has a long shelf life of over a year, without any special moisture-resistant packaging.
Successful 3D printing of PVDF has been achieved by selection and adjustment of three parameters in the 3-D process, with excellent results achieved by combining two or all three parameters. The parameters include: polymer or polymer blend selection, optional filler selection, and specific processing conditions.